1. Field of the Invention
Reciprocating down hole pumps are known for lifting fluids from wells. The reciprocating pump is located in the well casing in the area adjacent to the producing formation. The pump is actuated by a string of sucker rods extending to the surface. The sucker rod string is typically reciprocated by a surface "pumping unit" comprising a rocker arm that is rocked about a pivot mounted intermediate its ends on a vertical support structure by a Pitman rod driven by a prime mover. Usually, a counter weight is mounted upon the opposed limb of the rocker arm (hereinafter referred to as the "drive limb") to counter-balance the weight of the sucker rods and the piston of the pump. To pivot the rocker arm and thus to reciprocate the string of sucker rods vertically within the well, the upper end of a Pitman rod is fastened to the drive limb of the rocker arm. The lower end of this rod is driven by a rotating shaft from a gear box driven by the prime mover.
As applied to a typical oil well, the load on the prime mover is at a maximum when the rocker arm begins the upwardly movement of the suspended sucker rods. This load includes the weight of the sucker rods, the weight of the oil to be lifted and the force needed to overcome the inertia of the load. At this point the constant load permits the sucker rods to reach a constant velocity until the sucker rods approach the top limit of the pumping stroke and the upwardly movement ceases and the down stroke begins. The weight of the sucker rods accelerates the downwardly movement of the rocker arm until the sucker rods reach the bottom of the down stroke at which time the pumping cycle is repeated.
The loads imposed upon the sucker rods of an oil well pump jack are considerable. During the upstroke of a typical 5000 foot (1524 m.) well the weight of the sucker rods and the oil being lifted is approximately eight thousand pounds (2639 kg). The shock loadings placed upon the sucker rod as the sucker rod motion is reversed is considerable. The acceleration of the sucker rods is at a maximum at the extremities and the result can cause fractures thereof and resulting costly repairs of the sucker rods. The instant well pumping system reduces these shock loading and associated problems by employing a linear induction motor which imposes a maximum starting force on the sucker to overcome the inertia followed by a reduced force to overcome the needed lifting force for continuing the stroke and a braking force to decelerate the movement of the sucker rods until the top of the stroke. At the top of the stroke the weight of the sucker rods accelerates the downward movement of the sucker rods and the pump piston. At this point the induction motor is switched to operate as a generator, the output of which is used to charge a battery, or capacitor, this energy is either returned to the power supply or fed into the system for the next pumping stroke. Conventional counter-weights attached to the walking beam reduce the load required for the pumping stroke.
2. Description of the Related Art
A review of the related art discloses many proposed improvements in fluid pumps for reducing the shock loadings experienced by sucker rods reversing their motion. U.S. Pat. No. 4,461,187 Stanton, 1984, discloses a pump jack comprising a rocker arm one end of which is fixed to the upper end of a sucker rod. The other limb of the rocker arm has rigidly affixed thereto a downwardly extending drive support member and a large driven pinion is eccentrically and rotatably mounted at the lower end of this drive support member. This drive arrangement reduces the acceleration and shock loadings imposed upon the sucker rods at the beginning of the up stroke. U.S. Pat. No. 4,703,655 Thompson, 1987, discloses a pumping unit with a short base frame and interchangeable crank arm while retaining a relatively low net peak torque requirement obtained through a slow up stroke and a faster down stroke. U.S. Pat. No. 4,931,677 Gotz Heidelberg, 1990, discloses an electromagnetic linear drive including a stator which constitutes a patch of movement for a vehicle. The linear drive is equipped with current conductors and is divided into a plurality of stator elements of equal length. A movable portion which constitutes a vehicle is preferably equipped with permanent magnets cooperating with the stat or to provide the drive.